The present invention is generally related to system and method for controlling an electrical machine, and, more particularly, to system and method for controlling an induction machine using a slip boost technique during start up of the machine.
The advent of modern power electronics has in recent years dramatically changed machine and controller hardware used in traction control or electric vehicle applications. In the past, direct current (DC) machines have been very popular in such applications because of their generally linear torque response to armature current across a relatively wide range of speeds, and further because of the relative ease for respectively decoupling field and armature currents that enable independent control of either the field flux or the output torque to achieve a desired torque response. Unfortunately, DC machines generally suffer from lower machine efficiencies and the burdensome maintenance, speed limitations, and costs that accompany the use of conducting brushes and commutators. The disadvantages of DC machines have been reduced by the use of alternating current (AC) induction machines controlled by a suitable inverter, e.g., an insulated gate bipolar transistor (IGBT) or metal oxide semiconductor field effect transistor (MOSFET) inverter.
Propulsion drive systems for electric vehicles or hybrid electric vehicles need to have a relatively wide speed range, high torque per ampere, high efficiency, quick dynamic response, and operational robustness and reliability under tough environmental or operational conditions. As suggested above, induction machines have become widely used in such applications due to the advantages of their relatively low cost, reliability, and elimination of brushes and commutators. In particular, an induction machine is capable of four quadrant operation using a variable frequency and variable voltage drive control, such as a slip frequency control technique and a vector control technique. The use of vector control techniques in induction machines allows for decoupling the respective machine current components, such as a flux-producing component and a torque-producing component, to generate a machine response like that of a DC machine.
It is known that to reduce the cost of a propulsion system for applications such as Flywheel Alternator Starter (FAS) type hybrid vehicle, a relatively low resolution rotor position sensor or encoder would be more desirable than a high resolution rotor sensor due to the lower cost and higher reliability of the low resolution sensor. Since a high performance propulsion system generally requires a fine rotor position resolution, intelligent extrapolation techniques are being developed by the assignee of the present invention to increase the resolution available to a vector control module by extrapolating pulses in between any two successive coarse pulses of the low resolution encoder. This extrapolation technique has been found to work well across the required speed range, except during start up of the induction machine. In starting, the first several coarse pulses are generated under fast acceleration of the rotor, and this condition, somewhat prevents the extrapolation technique from producing sufficiently accurate rotor position information. The inability to produce accurate high resolution pulses during start up of the machine can result in unsuccessful cranking events if the electric machine cannot produce enough torque to overcome the compression and frictional forces of an internal combustion engine coupled thereto.
The synchronous angle boost technique of the present invention addresses the foregoing issues by providing estimated rotor position information during the first few coarse pulses. The inaccurate position information from the extrapolation technique is essentially discarded during start up and replaced by the estimated position information derived from the synchronous angle boost technique. After the extrapolation technique is able to produce accurate rotor position information, the synchronous angle boost technique is turned off.
Generally, the present invention fulfills the foregoing needs by providing in one aspect thereof, a method for controlling an induction machine. The method allows to sense rotor position of the induction machine using a relatively low-resolution rotor position sensor configured to supply a stream of pulses indicative of angular increments as the rotor position changes. A memory device is used for storing a synchronous angle boost function. The method further allows to retrieve from the memory device at least one function parameter for providing a synchronous angle boost during a start up mode of operation of the induction machine. Upon sensing a predetermined number of pulses from the rotor position sensor, the method allows to switch from the start up mode of operation to a normal mode of operation, wherein the boost provided to the synchronous angle is discontinued upon switching to the normal mode of operation.
The present invention further fulfills the foregoing needs by providing in another aspect thereof, a system for controlling an induction machine. The system comprises a rotor position sensor for sensing rotor position of the induction machine. The sensor supplies a stream of pulses indicative of relatively coarse angular increments as the rotor position changes. A memory device is provided for storing a synchronous angle boost function. A boost module is coupled to retrieve from the memory device at least one function parameter for providing a synchronous angle boost during a start up mode of operation of the induction machine. A switching module is configured to switch from the start up mode of operation to a normal mode of operation upon sensing a predetermined number of pulses from the rotor position sensor, wherein the switching module passes the boosted synchronous angle from the boost module during the start up mode, and wherein the boost provided to the synchronous angle is discontinued upon switching to the normal mode of operation.